Method for working nut screw for ball screw

ABSTRACT

A method for working a nut screw for a ball screw has a first step of lathing or cutting a spiral-shaped rolling surface of the nut screw for a ball screw on which balls of the ball screw are allowed to roll, a second step of heat treating, for example, carburizing and quenching or high-frequency quenching the spiral-shaped rolling surface obtained in the first step to thereby harden the top surface layer of the rolling surface and, a third step of surface finishing the hardened top surface layer of the spiral-shaped rolling surface obtained in the second step according to an electrolytic polishing.

TECHNICAL FIELD

The invention relates to a method for working a nut screw for a ballscrew, including a deflector-type ball screw.

BACKGROUND ART

A ball screw, which converts rotary motion into linear motion orconverts linear motion into rotary motion, not only can amplify smalltorque to thereby provide large thrust but also is capable of accuratepositioning in a linear direction. Thanks to such performance, the ballscrew is widely used in a machine tool, a semiconductor-associatedapparatus, and an industrial robot.

The ball screw, as shown in FIG. 4, includes a screw shaft 102 includinga spiral-shaped groove 101 and a nut screw 104 including a spiral-shapedgroove 103; and, a plurality of balls 105 are allowed to roll betweenthe spiral-shaped grooves 101 and 103 of the screw shaft 102 and nutscrew 104. The spiral-shaped groove 101 of the screw shaft 102 is formedby rolling work using a rolling die or by grinding into a Gothic-archshape. On the other hand, the spiral-shaped groove 103 of the nut screw104 is worked in the following manner.

That is, the blank material of the nut screw 104 is carburized andquenched, or is high-frequency quenched so as to have a given level ofhardness. Next, as shown in FIG. 5, the spiral-shaped groove 103 isfinished using a grindstone 107 mounted on the leading end portion of arotary shaft 106 so as to have a Gothic-arch shape.

However, in the above method for grinding the inside diameter of the nutscrew 104 using the grindstone 107 mounted on the leading end portion ofthe rotary shaft 106, the rotary shaft 106 must be formed so as to havea small diameter, so that the rotary shaft 106 is low in rigidity.Therefore, in the case of the thus-formed rotary shaft 106, it isdifficult to work the blank material that has been processed bycarburizing and quenching or by high-frequency quenching to have a givenlevel of hardness. Also, when finishing the spiral-shaped groove 103into a Gothic-arc-like spiral-shaped rolling surface 108, in case wherethe rolling surface 108 is not finished into a sufficiently smoothsurface, a surface peel-off phenomenon occurs between the rollingsurface 108 and the balls 105 rolling on the rolling surface 108 tothereby lower the life of the rolling surface 108. That is, it takes alot of time to grind the spiral-shaped groove 103 using the grindstone107.

Also, conventionally, there is used another method in which aspiral-shaped rolling surface, after completion of heat treatment, isfinished using a lapping operation instead of a grinding operation.However, in this method, lapping bars similar in shape to thespiral-shaped rolling surfaces and having grains attached thereto mustbe manufactured for every shapes of the rolling surfaces, which givesrise to an increase in the manufacturing cost of the nut screw.

Further, FIGS. 6 and 7 shows a nut screw 201 for a deflector-type ballscrew includes a spiral-shaped groove 202 formed in the inner peripheralsurface thereof. In the outer peripheral surface of the nut screw 201,there is formed a deflector fit hole 204 into which a deflector 203 canbe fitted and fixed. And, in order that the deflector fit hole 204 canbe fitted with the deflector 203 with no clearance between them, thedeflector fit hole 204 is worked so as to have the same shape as thedeflector 203.

And, a plurality of balls 207 are allowed to roll between thespiral-shaped groove 205 of a ball screw 206 and the spiral-shapedgroove 202 of the nut screw 201. Also, the balls 207 are disposed in theendless circulation passage 208 of the ball screw 206 and, in case wherethe nut screw 201 and ball screw 206 are rotated with respect to eachother, the balls 207 are allowed to roll on the endless circulationpassage through the circulation passage of the deflector 203.

By the way, conventionally, in a method for working the deflector fithole 204, using a cutting tool such as a drill or an end mill, the nutscrew 201 is cut from the outer peripheral surface side thereof.However, since a high thrust is required of the ball screw 206, the ballscrew 206 increases in size and also the number of circulation passagesincreases. That is, in the conventional working method, the cuttingremoval amount increases as well as the working time increases inproportion to the increase in the cutting removal amount.

Also, as means for fixing the deflector 203 strongly to the nut screw201, as shown in FIG. 8, a deflector fit hole 204′ to be formed in thenut screw 201 is structured in a dovetail groove which increases inwidth as it goes to the deep side thereof. And, after the deflector 203is fitted into the deflector fit hole 204′, the deflector 203 is caulkedand deformed using a punch 209, thereby preventing the deflector 203from slipping off the deflector fit hole 204′.

However, in the conventional working method for cutting the nut screw201 using a cutting tool such as a drill or an end mill, due to theincreased size of the ball screw and the increased number of circulationpassages, the cutting removal amount increases as well as the workingtime increases in proportion to the increase in the cutting removalamount. Also, in case where the deflector fit hole 204′ to be formed inthe nut screw 201 is structured in a dovetail groove which increases inwidth as it goes to the deep side thereof, there is necessary a toolwhich is specially designed for such hole working. This increases theworking time and thus gives rise to an increase in the working cost ofthe nut screw.

DISCLOSURE OF INVENTION

The present invention aims at eliminating the drawbacks found in theabove-mentioned conventional working methods of a nut screw for a ballscrew. Accordingly, it is an object of the invention to provide aworking method of a rolling surface of a nut screw for a ball screw,which can easily work the spiral-shaped rolling surface of the nut screwregardless of the size and shape thereof to thereby be able to shortenthe working time of the nut screw and thus reduce the cost thereof.

Further, it is another object of the invention to provide a method forworking a nut screw for a deflector-type ball screw, which not only canfacilitate the working of the deflector fit hole in the nut screw tothereby shorten the working time and thus reduce the working cost of thenut screw, but also can facilitate the fixation of the deflector to thedeflector fit hole to thereby reduce the working cost of the nut screw.

In attaining the above object, according to a first aspect of theinvention, there is provided a method for working a nut screw for a ballscrew, comprising: a first step of cutting the spiral-shaped rollingsurface of the nut screw on which balls of the ball screw are allowed toroll; a second step of heat treating the spiral-shaped rolling surfaceobtained in the first step to thereby harden the surface of the rollingsurface; and, a third step of surface finishing the surface hardenedlayer of the spiral-shaped rolling surface obtained in the second stepaccording to an electrolytic polishing.

Also, according to a second aspect of the invention, in the method asset forth in the first aspect of the invention, the first step works theabove-mentioned spiral-shaped rolling surface into a Gothic-arch shapeby lathing the spiral-shaped rolling surface using a lathe or by cuttingthe spiral-shaped rolling surface using a rotary tool.

As described above, according to the invention, by working the rollingsurface of the nut screw by lathing it using a lathe or by cutting itusing a specially designed rotary tool into a Gothic-arch shape, theworking time of the nut screw can be shortened and the working accuracythereof can be enhanced. Also, after the blank material of the nut screwis heat treated and is thereby surface hardened, by treating only therolling surface of the screw nut according to an electrolytic polishingmethod, the nut screw can be worked regardless of the size of the nutscrew and the shape of the rolling surface.

Further, in attaining the above object, according to a third aspect ofthe invention, there is provided a method for working a nut screw for adeflector-type ball screw, comprising the steps of: radiating a laserbeam onto a nut screw having a spiral-shaped groove formed in the innerperipheral surface thereof for allowing balls to roll thereon from theouter peripheral surface side of the nut screw to thereby form adeflector fit hole in the nut screw in such a manner that the deflectorfit hole penetrates through the nut screw from the outer peripheralsurface thereof up to the inner peripheral surface thereof; and, fittingand fixing a deflector for defining a circulation passage into thedeflector fit hole.

Also, according to a fourth aspect of the invention, there is provided amethod for working a nut screw for a deflector-type ball screw,comprising the steps of: forming a deflector fit hole in a nut screwhaving a spiral-shaped groove formed in the inner peripheral surfacethereof for allowing balls to roll thereon in such a manner that thedeflector fit hole penetrates through the nut screw from the outerperipheral surface thereof up to the inner peripheral surface thereof;fitting and fixing a deflector for defining a circulation passage intothe deflector fit hole; and, after then, welding the deflector to thenut screw using a laser beam.

According to the third aspect of the invention, since the deflector fithole is formed using a laser beam working operation, the materialremoval amount is limited to the contour passage of the deflector fithole. Thanks to this, when compared with a conventionally known cuttingoperation, the working time can be shortened greatly.

Also, according to the fourth aspect of the invention, the deflector iswelded to the nut screw using a laser beam working operation. This notonly can eliminate the need for formation of a specially-shapeddeflector fit hole for prevention of slippage of the deflector in thenut screw but also can omit the operation to caulk the deflector. Thanksto this, the working time can be shortened.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional side view of a ball screw, showing afirst embodiment of to the invention;

FIG. 2 is a side view of a working apparatus for working a nut screwused in a deflector-type ball screw, showing a second embodiment of theinvention;

FIG. 3 is a longitudinal sectional side view of part of a nut screw,showing a third embodiment of the invention;

FIG. 4 is a longitudinal sectional side view of a ball screw;

FIG. 5 is a longitudinal sectional side view of a conventional methodfor working the rolling surface of a nut screw;

FIG. 6 is a longitudinal sectional side view of a conventional nutscrew;

FIG. 7 is a longitudinal sectional side view of a conventionaldeflector-type ball screw; and,

FIG. 8 is a longitudinal sectional side view of part of a conventionalnut screw.

MODES FOR CARRYING OUT THE INVENTION

Now, description will be given below of the preferred embodiments of aworking method of a nut screw for a ball screw with reference to theaccompanying drawings.

FIG. 1 is a longitudinal sectional side view of a ball screw accordingto a first embodiment of the invention. The ball screw includes a screwshaft 2 including a spiral-shaped groove 1 and a nut screw 4 including aspiral-shaped groove 3; and, a plurality of balls 5 are allowed to rollbetween the spiral-shaped grooves 1 and 3 of the screw shaft 2 and nutscrew 4. The spiral-shaped groove 1 of the screw shaft 2, generally, isformed by a rolling work using a rolling die or by grinding so as tohave a Gothic-arch shape. On the other hand, the spiral-shaped groove 3of the nut screw 4 is worked using a lathe or is cut using a speciallydesigned rotary tool so as to have a Gothic-arch shape.

Next, the blank material of the nut screw 4 is heat treated and isthereby surface hardened, for example, by carburizing and quenching, orby high-frequency quenching so that the rolling surface 8 of the nutscrew 4 has a given level of hardness. In this state, however, therolling surface 8 is not sufficiently smooth; and, therefore, after thesurface hardening heat treatment of the nut screw 4, only the rollingsurface 8 is surface modified according to an electrolytic polishingmethod so that the rolling surface 8 has a desired surface roughness.

In the electrolytic polishing method used in the present invention,while the nut screw 4 masked except for the rolling surface 8 to bepolished electrolytically is considered as a positive electrode, in casewhere the positive electrode (the nut screw 4) is immersed together witha negative electrode in electrolyte and a direct current is allowed toflow through the two electrodes, the top surface of the rolling surface8 is gradually caused to melt away, with the result that the top surfaceof the rolling surface 8 is turned into a new smooth surface.

In this manner, in case where the rolling surface 8 of the nut screw 4is lathed using a lathe or is cut using a specially designed rotary toolinto a Gothic-arch shape, the working time of the nut screw 4 can beshortened and the working accuracy thereof can be enhanced. Also, afterthe surface hardening heat treatment of the nut screw 4, by treatingonly the rolling surface 8 according to an electrolytic polishingmethod, the nut screw 4 can be worked regardless of the size thereof andthe shape of the rolling surface 8.

FIG. 2 is a side view of an apparatus for working a nut screw used in adeflector-type ball screw according to a second embodiment of theinvention. The nut screw working apparatus includes a work supportmechanism 11 and, in the work support mechanism 11, there is disposed achuck 13 for gripping a nut screw 12. The chuck 13 can position the nutscrew 12 in the peripheral direction thereof and in the axial directionthereof using an NC (numerically controlled) index 14.

In the outer peripheral surface of the nut screw 12, there is formed alaser radiation port 16 of a laser working machine 15 in such a mannerthat it extends at right angles to the axis of the nut screw 12; and, alaser beam L, which is radiated from the laser radiation port 16, isapplied to the outer peripheral surface of the nut screw 12 to therebybe able to form a deflector fit hole 17 which penetrates through the nutscrew 12 from the outer peripheral surface thereof up to the innerperipheral surface thereof.

That is, at the same time when the chuck 13 moves the nut screw 12 inthe peripheral direction thereof and in the axial direction thereofusing the NC (numerically controlled) index 14, the laser beam L fromthe laser radiation port 16 is radiated to the outer peripheral surfaceof the nut screw 12. Therefore, the laser beam L is scanned along thecontour passage of the deflector fit hole 17 to thereby form suchdeflector fit hole 17 in the outer peripheral surface of the nut screw12 that penetrates through the nut screw 12 from the outer peripheralsurface thereof up to the inner peripheral surface thereof. Into thedeflector fit hole 17, similarly to the conventional structure, therecan be fitted a deflector; and, the deflector can be fixed to the nutscrew 12 by caulking or by screwing, or using a laser beam weldingoperation which provides a third embodiment of the invention to bediscussed below.

In case where the deflector fit hole 17 is formed using the laser beamworking operation in this manner, differently from the conventionalcutting operation, the material removal amount by the laser beam islimited to the contour passage of the deflector fit hole 17, which makesit possible to shorten the working time greatly. Therefore, the secondembodiment is effective in dealing with the increased size of the ballscrew and the increased number of circulation passages.

Now, FIG. 3 is a longitudinal sectional side view of part of a nut screw12 according to a third embodiment of the invention. In the nut screw12, there is formed a deflector fit hole 18. And, the deflector fit hole18 is wide in width on the outer peripheral portion side of the nutscrew 12 and is narrow in width on the inner peripheral portion side ofthe nut screw 12, whereby there is formed a fitting stepped portion 19in the deflector fit hole 18.

Also, a deflector 20 has a shape corresponding to the shape of thedeflector fit hole 18 and includes an engaging edge portion 21 which canbe engaged with the fitting stepped portion 19. Further, on the two sideedge portions of the deflector 20, there are formed chamfered portions22.

And, to fix the deflector 20 to the deflector fit hole 18, the deflector20 may be fitted into the deflector fit hole 18 to thereby bring thefitting stepped portion 19 into engagement with the engaging edgeportion 21. In this state, a laser beam L may be radiated from the laserradiation port 23 of the laser working machine to the connecting portionbetween the deflector fit hole 18 and deflector 20 to thereby laser weldthe deflector 20 to the deflector fit hole 18, so that the deflector 20can be fixed to the nut screw 12.

In this case, the portion to be heated and welded by the laser beam L islimited to a very small area, and the heated and welded portion can behardly deformed due to heat. Also, there is eliminated the need forworking of a taper hole for prevention of the deflector against slippagewhich can occur in the case of the conventional caulking operation,thereby being able to shorten the working time greatly.

While only certain embodiments of the invention have been specificallydescribed herein, it will apparent that numerous modifications may bemade thereto without departing from the spirit and scope of theinvention.

INDUSTRIAL APPLICABILITY

A working method of a nut screw of a ball screw of the invention, bylathing or cutting the blank material of the nut screw prior to the heattreatment thereof to thereby form the rolling surface of the nut screw,the working efficiency of the nut screw can be enhanced and the workingtime thereof can be shortened.

Also, after the blank material of the nut screw is heat treated andsurface hardened, by treating only the rolling surface of the nut screwaccording to an electrolytic polishing method, not only the nut screwincluding the rolling surface can be worked regardless of the size ofthe nut screw and the shape of the rolling surface, but also the workingcost of the nut screw can be reduced.

In addition, since the deflector fit hole is formed according to a laserbeam working operation, the material removal amount is limited to thecontour passage of the deflector fit hole. Thanks to this, when comparedwith the conventionally used cutting operation, the invention not onlycan shorten the working time greatly but also is effective in dealingwith the increased size of the ball screw and the increased number ofcirculation passages.

Also, because the deflector is welded to the nut screw using a laserbeam, there is eliminated the need for formation of a specially-shapeddeflector fit hole for prevention of slippage of the deflector in thenut screw, and there is eliminated the need for execution of theoperation to caulk the deflector. Thanks to this, the working time canbe shortened and thus the working cost can be reduced.

1. A method for working a nut screw for a ball screw, comprising: afirst step of cutting a spiral-shaped rolling surface of the nut screwon which balls of the ball screw are allowed to roll; a second step ofheat treating the spiral-shaped rolling surface obtained in the firststep for surface hardening to thereby obtain a surface hardened layer onthe rolling surface; and, a third step of surface finishing the surfacehardened layer obtained in the second step according to an electrolyticpolishing.
 2. The working method according to claim 1, wherein theelectrolytic polishing is performed by masking a portion of the nutscrew other than the rolling surface so as to serve as a positiveelectrode.
 3. The working method according to claim 1, wherein the firststep works the spiral-shaped rolling surface into a Gothic-arch shape bylathing the spiral-shaped rolling surface using a lathe or by cuttingthe spiral-shaped rolling surface using a rotary tool.
 4. A method forworking a nut screw for a deflector-type ball screw, comprising thesteps of: radiating a laser beam onto an outer peripheral surface sideof the nut screw having a spiral-shaped groove formed in the innerperipheral surface thereof for allowing balls to roll thereon to therebyform a deflector fit hole in the nut screw in such a manner that thedeflector fit hole penetrates through the nut screw up to the innerperipheral surface of the nut screw; and, fitting and fixing a deflectorfor defining a circulation passage into the deflector fit hole.
 5. Theworking method according to claim 4, further comprising the steps of:cutting the spiral-shaped groove; heat treating the cut spiral-shapedgroove for surface hardening to thereby obtain a surface hardened layeron the spiral-shaped groove; and, surface finishing the surface hardenedlayer according to an electrolytic polishing.
 6. A method for working anut screw for a frame-type ball screw, comprising the steps of: forminga deflector fit hole in the nut screw having a spiral-shaped grooveformed in the inner peripheral surface thereof for allowing balls toroll thereon in such a manner that the deflector fit hole penetrates thenut screw from the outer peripheral surface thereof up to the innerperipheral surface thereof; fitting a deflector for defining acirculation passage into the deflector fit hole; and, welding saiddeflector to said nut screw using a laser beam.